(a) The Field of the Invention
The present invention relates to a process for the preparation of Decabromodiphenyl ether. More particularly, the present invention relates to a process which employs a mixture of halogenated organic solvents, by which Decabromodiphenyl ether is obtained, having improved thermal stability.
Decabromodiphenyl ether, hereinafter referred to as "DECA" for the sake of brevity, is a well known flame retardant agent, useful in the preparation of articles made of polymeric material, to which it is desired to impart flame-retardant properties.
(b) The Prior Art
According to the known art DECA is prepared in a variety of solvents, ranging from liquid bromine to halogenated organic solvents. U.S. Pat. No. 4,521,633 discloses one of such processes in which DECA is prepared by reacting diphenyl ether in methylene chloride (dichloromethane) with a brominating agent in the presence of a catalyst, by initiating the reaction at a temperature of 15.degree. C. or lower, and then raising the temperature of the reaction mixture to an elevated temperature.
U.S. Pat. No. 3,959,387 discloses a process for the preparation of polybrominated biphenyl oxides in which the reaction is carried out in methylene bromide as the solvent, at temperatures of from room temperature to 200.degree. C.
The above and other processes according to the art employ substantially pure halogenated solvents, such as substantially pure methylene chloride or methylene bromide. When organic solvents are employed for the preparation of DECA, it is a generally accepted principle in the art that the solvent must be a substantially pure solvent. Thus, for instance, U.S. Pat. No. 4,521,633 states that the use of methylene chloride is particularly advantageous in that it exhibits very low susceptibility to transhalogenation. Similarly, U.S. Pat. No. 3,959,387 teaches that the use of methylene bromide as the solvent is necessary to the conduction of the reaction.
The use of pure chlorine-containing solvents, according to the art, has the considerable drawback of requiring costly and time-taking purification steps, because of the transhalogenation that takes place during the reaction with solvents such as methylene chloride. Reaction of diphenyl oxide with a brominating agent in methylene chloride in the presence of a bromination catalyst inevitably causes some transhalogenation of the solvent to take place, which results in the presence of measurable amounts of both bromochloromethane and dibromomethane in the reaction mixture. These transhalogenation products must be separated before the methylene chloride is reused in a subsequent run.